Block and graft copolymers of silicone and vinyl monomers combine the properties of two dissimilar polymers providing unique materials which potentially have broad utility in a number of areas including release coatings for pressure-sensitive adhesives, and permselective membranes. Such polymers find use in applications that require the combination of the unique properties of the silicone with those of the vinyl polymer. Such applications include permselective membranes (Mueller et al., U.S. Pat. No. 4,486,577), photoresists (U.S. Pat. No. 4,689,289 to Crivello), electron beam resists, (M. J. Bowden, et al., Polymeric Materials Science and Engineering, 55, 298 (1986)), and release coatings for pressure sensitive adhesives (Clemens, U.S. Pat. No. 4,728,571). According to the teaching of Clemens, the copolymer should contain little or no unincorporated silicone, otherwise loss of adhesive performance is observed.
General methods of preparing such materials have only recently been described. Kawakami (Makromol. Chem., 185 9 (1984)), teaches the preparation of a vinyl polymeric backbone grafted with silicone polymeric segments in a comb-like structure by copolymerizing a silicone macromonomer with vinyl monomers.
A thermal method involving the use of a macromolecular siloxane initiator is disclosed in Crivello U.S. Pat. No. 4,584,356. Crivello utilizes a macromolecular siloxane thermal initiator which, due to termination by coupling of the growing polymer radicals, leads to a segmented (AB).sub.n block copolymer. Crivello describes utility of the copolymer thus formed as E-beam resists (U.S. Pat. No. 4,677,169) and positive and negative resists (U.S. Pat. No. 4,689,289).
A photochemical method involving the use of a macromolecular siloxane iniferter is described in concurrently filed copending U.S. patent application Kumar, et al., Siloxane Iniferter Compounds, Block Copolymers Made Therewith and a Method of Making the Block Copolymers (assigned to the assignee of the present case). Kumar, et al., teaches polymerization of vinyl monomers with a novel siloxane "iniferter" which, unlike Crivello, yields ABA or AB type block polymers due to the presence of a reactive terminating radical which minimizes coupled between the two polymeric radicals.
These preparative methods are not without their drawbacks. Crivello's method requires an elaborate synthesis of the siloxane initiator and based on the mechanism presented by Crivello (JPS, Poly. Chem, 24, 197 & 1251 (1986)), initiator fragments exist between the siloxane and vinyl blocks, creating a weak link which on aging may release free silicone. The photoiniferter method generates quality material which performs very well when tested as a release coating. However, because of the photopolymerization required, some potential producers are ill-equipped to manufacture this material in a cost-effective manner.
Inoue and Kohama (Kagaku to Kogye (Osaka), 1986, 60 (3), 81 (Chem Abs. 105:1735345)) discloses the grafting of vinyl monomers to a silicone backbone to yield a comblike structure with the roles of silicone and vinyl reversed from those of Clemens, wherein grafting occurs concomitantly with gel formation and wherein UV radiation is utilized to initiate polymerization.
Block polymers are also prepared from silicone macroinitiators containing labile peroxy ester (JP 63 57,642, publication date Mar. 12, 1988, and 57,644, publication date Mar. 12, 1988,) or azo (Inoue and Ueda, Journal of Applied Polymer Science, 35, 2039, (1988)) functionality, or by the coupling of a preformed monofunctional vinyl polymeric segment with a difunctional siloxane. In this latter case, the use of a mono hydroxy (DE 3,606,984), publication date Sept. 10, 1987, or carboxy (DE 3,606,983), publication date Sept. 10, 1987, terminal vinyl polymeric segment precludes the presence of similar functionalities on the polymer which would interfere with the coupling reaction.
Saam and Tsai, Journal of Applied Polymer Science, 18, 2279-2285 (1974), have utilized silicone modified with mercapto alkyl side groups to prepare stable dispersions during the free-radical polymerization of methyl methacrylate in aliphatic hydrocarbons. The silicone is grafted with vinyl monomer via a chain transfer reaction, with less than 50% of the charged silicone being grafted. This is due to high degree of insolubility of polydimethylsiloxane (PDMS) in the polymethyl methacrylate (PMMA) particles. As a result, the reactive sites on the dissolved stabilizer precursor are inaccessible to the growing polymeric radicals during the later stages of polymerization.
Mercaptoalkyl functional silicones are commercially available and find use as additives in vulcanized elastomers (Pape and Stewart Rubb Chem. Tech. 46, 585, (1973)), for preparation of curable siloxanes by Michael reaction with multifunctional acrylates (U.S. Pat. No. 4,725,630), for curing without modification (U.S. Pat. No. 4,239,674), and for cocuring with vinyl functional siloxanes (Eckberg et al., Canadian Patent 1233290).
A number of references describe the preparation of mercapto-functional silicone compounds. U.S. Pat. No. 4,238,393 describes the preparation of organopolysiloxanes containing one or more pendant or terminal mercapto groups by, for example, an acid-catalyzed co-equilibration reaction of a hydrosylate of dialkoxy mercaptoalkyl silane with octamethylcyclotetrasiloxane and hexamethyldisiloxane endblocker and its use as an additive in coating compositions to impart anti-blocking behavior.
U.S. Pat. No. 4,046,795 describes a process for the preparation of thiofunctional polysiloxane polymers by reacting a disiloxane and/or a hydroxy or hydrocarbonoxy containing silane or siloxane with a cyclic trisiloxane in the presence of an acid catalyst wherein at least one of the above organosilicon compounds contain a thiol group.
U.S. Pat. No. 4,783,490 describes the preparation of mercapto substituted silicon compounds by amidizing an aminofunctional silicon compound with a mercapto carboxylate.
Canadian Patent No. 1,233,290 describes the preparation of mercapto-alkoxyalkyl-functional siloxanes by displacement of a haloalkyl functional siloxane with a hydroxyl functional thiol.
The above mercapto functional silicone references do not teach the use of mercapto functional silicones as chain transfer agents in the preparation of vinyl silicone copolymers.
Tung, U.S. Pat. No. 4,169,115 teaches the preparation of terminally mercaptofunctional anionic polymers (e.g., polystyrene, polybutadiene, etc.) and their use as macromolecular chain transfer agents in free radical polymerizations in order to prepare block copolymers. Tung does not teach the preparation of vinyl-silicone copolymers.
Thus, a need therefore exists for a convenient cost effective general method of synthesizing high quality vinyl-silicone copolymers which does not require expenditures for expensive equipment.
As indicated above, copolymers of silicone and vinyl monomers have potential utility in the area of release coatings for pressure sensitive adhesives. Normally tacky and pressure-sensitive adhesive (PSA) materials have been used for well over half a century. Products of this type, which take the form of tapes, labels, and other types of adhesive coated sheets, must be protected from unintended adhesion to other surfaces. Hence, tapes are typically wound into a roll on their own backing and labels are typically laminated to a release sheet to prevent their accidental adhesion to other surfaces and also to prevent their contamination with air-borne dust and other contaminants. In order to allow the roll to be unwound without the undesirable transfer of adhesive to the tape backing, it is customary to provide the tape backing with a low adhesion backsize (LAB). Similarly, the release sheet or liner to which the adhesive coated label is typically laminated is supplied with a release coating to permit the easy removal of the liner from the label.
This LAB or release coating is expected to reproducibly provide an appropriate level of release to the adhesive of interest, to not deleteriously affect the adhesive, and to be resistant to aging so that the release level remains relatively stable with time. In recent years, as competition has expanded in the PSA industry, a need to differentiate product performance as well as more demanding product requirements has lead to recognition of the importance of release level. D. Satas, Chapt. 23 "Release Coatings", Handbook of Pressure Sensitive Adhesive Technology, Second Edition, D. Satas, ed., Van Nostrand Reinhold, 1989, defines seven distinct levels of release, ranging from "super low release" (0.15-0.30 N/dm) to "very tight release" (20-80 N/dm). Even within a given category, such as "moderately tight release" (6-10 N/dm), consumer preference demands a tighter unwind for a roll of office tape in Europe and Japan than in North America, allowing the manufacturer who has a means of easily adjusting the level of release the opportunity to compete on a global basis.
In many circumstances it is important for the LAB to possess other properties besides functioning as a release agent. For example, the release coating on masking tape must possess good solvent resistance in addition to providing a surface to which paint can adhere.
Polymers with long straight chain hydrocarbon branches find widespread use as low adhesion backsizes for pressure sensitive tapes. The alkyl side chain of these acrylate (U.S. Pat. No. 2,607,711), methacrylate (U.S. Pat. No. 3,502,497 and U.S. Pat. No. 4,241,198) vinyl ester (U.S. Pat. No. 2,829,073), vinyl carbamate (U.S. Pat. No. 2,532,011), etc., copolymers apparently crystallizes to form a waxy low energy surface to which the adhesive adheres poorly. These various polymeric release coatings are not universal, in that none of them show desirable release performance for every type of PSA. In addition, the range of release level possible with these polymers is fairly limited.
Fluorocarbon copolymers also provide low surface energy coatings which find utility in certain specialty applications (U.S. Pat. No. 3,318,852), but also lack universality and release level tailorability.
Silicones are widely used for release liner applications due to the fact that they provide easy release for a wide variety of PSA types. Silicones are generally less useful as LABs in tape constructions, however, where tighter release levels are desirable. To increase their usefulness in LAB tape constructions, organosiloxane has been blended with a variety of film forming coating materials such as nitrocellulose (U.S. Pat. No. 2,985,554), alkyl ether-maleic anhydride copolymers (U.S. Pat. No. 3,770,687 and U.S. Pat. No. 3,823,025), vinyl alkyl carbamate copolymers (U.S. Pat. No. 3,679,458), etc., to tighten release. In addition silicones can be modified with epoxy groups to make them more polar (U.S. Pat. No. 4,313,988) hence providing tighter release. Silicones can also be cocured with isocyanates (U.S. Pat. No. 3,957,724), polybutadiene (U.S. Pat. No. 4,261,876), acrylic emulsions (U.S. Pat. No. 3,933,702), etc., to tighten release.
Release coating compositions based on polysiloxane grafted copolymers that do not require a curing step have been described (U.S. Pat. No. 4,728,571) (assigned to the assignee of the present case wherein controlled and predictable release is achieved through variation in the number and the length of the polysiloxane grafts.
Polymers having at least one polymeric siloxane segment and at least one hydrophilic vinyl polymeric segment prepared by other means than by use of the so-called chain-transfer method have been shown to demonstrate utility as release coatings that are capable of being written on effectively with water and oil based pen inks as described in copending U.S. patent application Mertens, Ser. No. 07/278,283, filed Nov. 30, 1988, (assigned to the assignee of the present case). Mertens does not teach a method for the preparation of vinyl siloxane block or graft copolymers by use of a mercapto functional silicone chain transfer agent nor does it disclose the use of materials prepared by such a method as general purpose release coatings.
An article by Noshay and McGrath, entitled "Block Copolymers", Academic Press, New York, 1977 pp. 156-162, which is hereby incorporated by reference, discusses phase separation as it applies to silicone-vinyl block copolymers. Noshay does not teach the preparation of silicone-vinyl block copolymers by the use of a mercapto functional silicone compound chain transfer agent.
Copending, concurrently filed, U.S. application 07/393,550, Kumar et al, Siloxane Iniferter Compounds, Block Copolymers Made Therewith, and a Method of Making The Block Copolymers, discussed previously, teaches the utilization of a novel siloxane iniferter compound as a means of promoting, controlling and terminating polymerization of a vinyl-siloxane block copolymer.
Copending, concurrently filed, U.S. application 07/393,557, Kumar, et al, General Purpose Siloxane Release Coatings (also assigned to the assignee of the present case) teaches the use of the iniferter prepared vinyl-siloxane block copolymers as release coatings.
However, none of the aforementioned references teaches a method of preparing a soluble vinyl-silicone block copolymer or graft copolymer utilizing a mercapto functional silicone compound chain transfer agent wherein the resultant copolymer has utility as a release coating for pressure sensitive adhesives.
Thus, in addition to the need which exists for a cost-effective, convenient route of synthesis for vinyl-silicone copolymers, a need also exists for sheet materials having release coatings of conveniently prepared vinyl-siloxane copolymers and tape constructions having low adhesion backsizes of the same copolymers. The copolymers which, in addition to being reliably produced, should exhibit specific release properties toward tacky and pressure-sensitive adhesives throughout a broad range.